Inverters are well known devices for providing AC power to a load such as a motor from a DC source such as rectified source.
Referring to FIG. 1, a simple inverter configuration is shown at 100 as including an insulated gate bipolar transistor (IGBT) 102, diode 104 and gate driver 106. The configuration is well known to the skilled person and not described in detail here and can include multiple diodes, IGBTs and drivers.
According to known systems the peak or absolute temperature of the device is maintained below a fail temperature for example 150° C. or a threshold temperature below that, for example 135° C. in order to avoid degraded performance or failure. One known device is the Unidrive M700 product available from Control Techniques Ltd. in which, as can be understood from FIG. 1 in conjunction with FIG. 2 temperature feedback of the case is obtained by a module such as a thermistor 108 which feeds the value to a temperature monitoring and pulse width modulation (PWM) controller 110 which receives additional measured operating conditions and control parameters to model device temperature as described below. In particular the temperature can be reduced by reducing the switching frequency of the inverter via the controller 110. The process can be further understood from FIG. 2 in which device junction-to-case temperature is derived as a function of measured operating conditions 200 including DC link voltage and device current which is fed to a power loss module 202 together with the current switching frequency from controller 204 and optionally further control parameters such as modulation depth acting effectively as a further model parameters. In conjunction with a thermal resistance model 206 a peak temperature is calculated at 208 in conjunction with the output frequency from motor control algorithm 210 to compensate for variation of power loss with time to provide a more accurate representation of peak temperature as a function of output frequency. Together with case temperature feedback 212 from thermistor 108 this is summed at 214 to obtain a temperature assessment 216 of junction-to-case temperature plus case temperature, ie absolute temperature rise which is fed to the switching frequency control. The switching frequency is fed back to the power loss module and also to the PWM controller 110 which generates appropriate gate signals 218 to control the switching frequency of the inverter. The approach prolongs the lifetime of switching devices by minimising the power loss through reduction of the switching frequency and in particular prevents the device shutting down when the fail temperature is met. However known devices do not operate at optimum performance according to this approach and can still, for example, face a reduced lifetime.